1. Technical Field
The present invention relates to a telescopic shaft. Particularly, the present invention relates to a telescopic shaft capable of transmitting a rotational torque and relatively movable in an axial direction, such as a telescopic shaft of an intermediate shaft and a steering shaft. Further, the invention relates to a steering apparatus having a telescopic shaft.
2. Background Art
In a steering apparatus, a telescopic shaft capable of transmitting a rotational torque and connected relatively movably in an axial direction is incorporated in an intermediate shaft, a steering shaft or the like. That is, an intermediate shaft needs to be provided with a telescopic function by which when an universal joint is fastened to a pinion shaft brought in mesh with a rack shaft of a steering gear, the intermediate shaft is temporarily contracted and thereafter fitted to the pinion shaft to fasten.
Further, a steering shaft needs to transmit a steering force of a steering wheel to a wheel and adjust a position of the steering wheel in an axial direction in accordance with a physical constitution or a driving attitude of a driver. Thus, a telescopic function is requested therefor.
In order to realize an excellent operability of the steering wheel with the telescopic shaft, it is necessary that backlash in a rotational direction between a male shaft and a female shaft slidable relative to each other is small and a sliding resistance in the axial direction between the male shaft and the female shaft is maintained at a predetermined sliding resistance over a long period of time. Such a telescopic shaft is disclosed in Japanese Patent Document 1 (JP-A-11-208484) and Japanese Patent Document 2 (JP-A-5-116633).
A telescopic shaft of Patent Document 1 is constituted by a male shaft having a noncircular outer periphery, a female shaft having a noncircular inner periphery, and an elastically deformable slide sleeve inserted to a gap between the outer periphery of the male shaft and the inner periphery of the female shaft and having a constant wall thickness. The gap between the outer periphery of the male shaft and the inner periphery of the female shaft is changed along a peripheral direction. Thus, the slide sleeve alternately comes in contact with the outer periphery of the male shaft and the inner periphery of the female shaft. The slide sleeve is bent as contact portion constitutes a fulcrum, and a preload is applied to between the male shaft and the female shaft.
According to the telescopic shaft of Patent Document 1, when fabrication errors of the outer periphery of the male shaft and the inner periphery of the female shaft are increased, an amount of bending the slide sleeve is changed such that the preload is varied. Therefore, the backlash in the rotational direction or the sliding resistance between the male shaft and the female shaft is deviated from a predetermined value. Further, when the contact portion of the slide sleeve is worn by a telescopic motion between the male shaft and the female shaft, the preload is reduced such that the backlash in the rotational direction between the male shaft and the female shaft is increased.
A telescopic shaft of Patent Document 2 comprises a male shaft having a noncircular outer periphery, a female shaft having a noncircular inner periphery, and two wedge pieces interposed between the outer periphery of the male shaft and the inner periphery of the female shaft. Further, two wedge pieces are interposed at a gap between two slide bushes integrally formed with the male shaft and are urged in directions separated from each other by a spring.
According to the telescopic shaft of Patent Document 2, a number of parts is so large that a fabrication cost is increased and fabrication errors of parts accumulates. Thus, it is difficult to achieve backlash in a predetermined direction or a slide resistance. Further, owing to a structure of arranging the slide bushes at both end portions in the axial direction of the male shaft and arranging the wedge pieces on inner sides thereof, the both end portions in the axial direction of portions of fitting the male shaft and the female shaft are operated with the preload by the wedge pieces. Therefore, it is insufficient to exclude backlash in a fold-to-bend direction (moment direction), and when a spring force is strengthened in order to exclude backlash in the fold-to-bend direction (moment direction) the sliding resistance becomes excessively large.